I respectfully disagree with your interpretation. Basically, I'm assuming that your document is pre-approved language. If so, almost certainly it will have some sort of "fail-safe" language. For example, that if the Eligible Employee does not complete the stated hours of service requirement in the specified time period (in your case, 520 hours in the 3-month period) they will become subject to the 1 Year of service requirement. Which of course, is 1,000 hours.
Where in 410(a) do you see anything where the IRS indicates that 520 hours in 3 months is unreasonable or unduly restrictive? As long as the plan is written such that you can't violate the age 21/1 YOS standard (or 2 years if 100% vested) you should be fine.
I'm oversimplifying here, I know that...but I don't want to delve into every possible permutation!